Idiopathic pulmonary fibrosis (IPF) is increasing in prevalence, and the recently approved anti-fibrotic therapies have limited efficacy. Alveolar macrophages have a critical role lung injury and repair. Macrophages in chronic disease typically exhibit apoptosis resistance, and their prolonged survival is generally associated with disease progression due to polarization to a pro-fibrotic phenotype. The generation of mitochondrial ROS (mtROS) in alveolar macrophages modulates pro-fibrotic polarization; however, the molecular mechanism(s) regulating macrophage mtROS in fibrosis is not clearly defined. One of the NOX enzymes, NOX4, generates mtROS with various stimuli and in several cell types, but the modulation of the macrophage phenotype has not been linked to NOX4. Our preliminary data show that alveolar macrophages from IPF subjects express high levels of the NOX4 gene compared to normal subjects, and there is more NOX4 localized in the mitochondria of IPF alveolar macrophages. NOX1/4 inhibition with GKT137831 or silencing NOX4 significantly abrogates mtROS. Moreover, GKT137831 down regulates pro-fibrotic polarization of macrophages and abrogates fatty acid oxidation and oxidative phosphorylation, which is characteristic metabolism for pro-fibrotic macrophages. In vivo, NOX4 modulates the polarization of alveolar macrophages to a pro-fibrotic phenotype. In addition, NOX4-/- mice, which are protected from pulmonary fibrosis, have significantly less alveolar macrophages in the BAL fluid suggesting either a defect in recruitment or an absence of apoptosis resistance. Our hypothesis is that NOX4 modulates macrophage mtROS and metabolism to polarize alveolar macrophages to a pro-fibrotic phenotype that is critical for fibrosis development. We will test this hypothesis with three specific aims. Aim 1 will determine if the Nox1/4 inhibitor (GKT137831) modulates alveolar macrophage metabolism and phenotype in IPF subjects enrolled in a Phase IIb clinical trial. In Aim 2, we will determine the effects of NOX4 in regulating mitochondrial metabolism and alveolar macrophage phenotype using pharmacologic (GKT137831) and genetic approaches. Aim 3 will determine macrophage-specific roles of NOX4 in regulating fibrotic responses to lung injury utilizing mice harboring a conditional deletion of NOX4 in macrophages. These studies may delineate NOX4 as a critical regulator of metabolism and macrophage plasticity suggesting it is an ideal therapeutic target to halt progression or reverse pulmonary fibrosis.